Abstract:

Equivalent Effective Stratospheric Chlorine (EESC) is a parameter designed to estimate that component of ozone variability in the stratosphere that is driven by changes in the long-term abundances of Ozone Depleting Substances (ODSs) in the atmosphere. Currently available estimates of EESC from the past to the present are based on a combination of direct ODS measurements, ODS measurements on Antarctic firn/ice air and estimates of ODS abundances based on ODS emissions. The latter are also used to estimate future EESC values. In this paper we derive past (from about 1930) and present EESC values based entirely on atmospheric and Antarctic firn air measurements, avoiding the need to invoke ODS emissions estimates to ‘fill-in’ observational ‘gaps’. The EESC values we calculate based entirely on observations agree very well with the EESC values from a combination of observations and emission estimates published in the most recent (2011) international assessment of stratospheric ozone depletion, and also agree well with the observed variability of Antarctic ozone hole metrics over the past 35 years.